It is common knowledge that household fireplaces are generally inefficient as far as room heating is concerned. Most of the heat goes up the chimney rather than into the room. The heat generated in the conventional fireplace which does enter the room enters only by radiation. This heat is quite intense immediately in front of the fireplace; however, the air a few feet from the fireplace is hardly warmed. As a matter of fact, the use of a fireplace requires air being drawn into the fireplace to support the fire of the burning logs. At least a part of this fire supporting air is most often drawn into the fireplace through cracks existing around doors and windows in the room where the fireplace is located. This tends to produce an overall cooling of the room rather than a warming. Generally, therefore, fireplaces as found in private dwellings are used more for their aesthetic appearance than their heating ability.
Attempts have been made in the past to increase the efficiency of private dwelling fireplaces. In newly constructed homes, siphoned-cooled, prefabricated, self-contained units can be installed. These units are multi-walled with ducting for drawing cold air down the internal side walls of the chimney through the fire and out the chimney as flue air. Still, radiation is relied upon for room heating. With fireplaces of the more conventional type, fireplace grates with means for forcing air through the heated area of the fireplace have been used to increase heating efficiency. Such grates are also sometimes used in prefabricated fireplaces to further increase their heating efficiency.
In construction, fireplace grates with air flow structures typically include hollow ducting through which forced air is circulated and directed into the room in which the fireplace is located. The forced air circulated through the ducting is warmed by the heat of the fire and thus warm air is blown into the room. Grate constructions of this type have, however, certain disadvantages. Most of them are rather bulky in construction and aesthetically unpleasing in looks. In addition, and more importantly, the efficiency of these forced air grates is generally poor.
The internal ducting through which the air is forced for warming is located either above the burning logs or below the logs at a location where embers and ashes from the logs are collected. With the first arrangement, the air is blown into the room at a level which is a few feet off the floor. It will be apparent that this is not the best situation since the most efficient heating of the room will occur where the heat is blown in at or near floor level. In a room with people sitting on chairs and couches, and perhaps on the floor, warm air entering the room from the fireplace at a level of even 2 feet above the floor will not produce the best warming effect for the people.
In the other type of grate construction where the air ducting is located below the logs, such ducting is most usually provided by making the log supporting grate bars hollow. This type of structure is inefficient in a number of respects. First of all, embers and ashes from the burning logs fall through the grate bars and collect on the floor below. The covering of the embers by the falling ashes provides a barrier against efficient heat transfer from the embers to the air passing through the hollow grate bars. Also, with this type of construction, the burning embers which fall from the logs tend to become cooled by the air being blown through the hollow grate bars and thus do not burn completely. Furthermore, since these embers are scattered in the ashes below the hollow bars while the burning logs are disposed above the grate bars, the air circulating through the bars acts as a thermal barrier preventing the heat of the embers from supporting the combustion of the fire of the burning logs. Taking away this support heat for the burning logs, tends to put the fire out prematurely.